ROOSELYNA EKAWATI, FOU-LAI LIN and KAI-LIN YANG DEVELOPING AN INSTRUMENT FOR MEASURING TEACHERS’ MATHEMATICS CONTENT KNOWLEDGE ON RATIO AND PROPORTION: A CASE OF INDONESIAN PRIMARY TEACHERS Received: 9 September 2013; Accepted: 9 March 2014 ABSTRACT. In this study, we developed an instrument for assessing teachers’ mathematics content knowledge (MCK) on ratio and proportion and examined the profile of Indonesian primary teacher’s MCK on this topic. The MCK items were administered to 271 Indonesian in-service primary teachers with a variety of educational backgrounds and teaching experiences. Teachers’ responses were analyzed by factor analysis and cluster analysis. The MCK instrument was found to have good acceptability in the reliability analysis with 3 factor components—meaning of proportional and non-proportional situations, number structures in situation, and figural representation—which was the main result of the study. With respect to the 3 factors, the teachers in the 3 assigned categories (“Good,”“Middle,” or “Low”) showed consistent performance on the items of the 3 factors. In particular, our results indicated that Indonesian in-service primary teachers had difficulty with the factor on figural representation, but they performed best on number structures in situation representing products of proportional reasoning. KEY WORDS: Indonesia, mathematics content knowledge, proportional reasoning, ratio and proportion INTRODUCTION In the last two decades, there has been increased attention to teachers’ knowledge of mathematics. On the essentials of mathematics content knowledge, Ball, Hill & Bass (2005) asserted: “How well teachers know mathematics is central to their capacity to use instructional materials wisely, to assess students’ progress, and to make sound judgments about presentation, emphasis, and sequencing” (p. 14). More specific attention was paid to the conceptual understanding of mathematics as Fennema & Franke (1992) found its positive relation with classroom instruction. Another important finding was the strong connection between the knowledge of potential future teachers and students’ achievement in the Teacher Education and Development Study in Mathematics (TEDS-M) Electronic supplementary material The online version of this article (doi:10.1007/s10763-014- 9532-2) contains supplementary material, which is available to authorized users. International Journal of Science and Mathematics Education 2014 # National Science Council, Taiwan 2014